1. Field of the Invention
The present invention relates to a probehead of a magnetic resonance (MR) apparatus used to transmit an RF field and to receive an MR signal in an MR apparatus such as an MR imaging apparatus and an MR spectroscopy apparatus.
2. Description of the Related Art
A conventional probehead of an MR apparatus used in a medical diagnosis is arranged to have an equivalent circuit diagram shown in FIG. 1. This probehead comprises a transmitting coil L1, a receiving coil L2, and variable capacitors C1 to C6. In order to protect a function of the probehead and to maintain its signal-to-noise (S/N) ratio, one of the transmitting and receiving coils L1 and L2 which is currently not used must be decoupled. That is, upon transmission, the transmitting coil L1 for forming an RF field from the probehead toward an object to be examined is tuned, while the receiving coil L2 for receiving an MR signal based on an MR phenomenon of the object to be examined is decoupled. Upon reception, to the contrary to the transmission, the transmitting coil L1 is decoupled, and the receiving coil L2 is tuned. Such decoupling control upon transmission and reception is performed by a switching section 2A connected to each of the transmitting and receiving coils L1 and L2.
A PIN diode is used as a switching element for performing a decoupling operation of the coils L1 and L2 at high speed.
The following description will be limited to a transmitting coil for handling high RF power.
FIG. 2 shows a circuit of a transmitting system of a probehead using a PIN diode D.
The transmitting circuit shown in FIG. 2 comprises RF (choke) coils RFC1 and RFC2 and a diode driver 31 in addition to the variable capacitors C1, C2, and C3, the transmitting coil L1 and the PIN diode D. The driver 31 includes DC bias power supplies 32 and 33 and a switch SW.
The PIN diode D is obtained by inserting an I region (intrinsic region: an intrinsic semiconductor region having a sufficiently low impurity concentration) between a p- and n-type regions of a p-n junction diode. In the PIN diode D, an RF series resistance changes upon application of a forward DC bias current. In general, a single PIN diode D is used in a conventional probehead as shown in FIG. 2.
In a probehead for transmitting an intense RF field, however, it is difficult to control decoupling of the transmitting coil L1 by the single PIN diode D. The reasons for this are: (1) an RF power to be applied to the transmitting coil L1 is significantly high (high RF current is supplied to the coil L1); (2) the PIN diode D is preferably connected in series with the transmitting coil L1 in order to decouple the coil L1 within a wide range including the resonant frequency; and (3) since the transmitting coil L1 is a kind of a resonant circuit and therefore a large resonant current flows through each circuit element, it is very difficult to form an arrangement in which only the single PIN diode D can flow the RF current in a resonant mode. Therefore, the present inventor assumed that in a probehead for transmitting an intense RF magnetic field, a plurality of PIN diodes D1, D2, ..., DN must be connected in parallel with each other to arrange the switching section 2A as shown in FIG. 3.
When the PIN diodes D1, D2, ..., DN are simply connected in parallel with each other as shown in FIG. 3, however, an RF current Io concentrically flows to a specific PIN diode (e.g., D1) having a small ON resistance due to variations in ON resistances of the diodes D1, D2, ..., DN, thereby destroying this PIN diode.
On the contrary, when current-limiting resistors are connected in series with the PIN diodes D1, D2, ..., DN in order to prevent destruction of the PIN diode, destruction of the PIN diode can be prevented. However, the current-limiting resistors for balancing produces thermal noise. For this reason, such a current-limiting resistor cannot be used to prevent destruction of the PIN diode.